I recently planted a batch of cucumber seeds. keeping them inside they germinated well and had grown to a few inches tall with two main leaves and the next set of leaves sprouting. Gradually they have all gone white mid stem, before the stem had thinned and collapsed. This happened both before I moved them into my outdoor mini greenhouse and since.

Thank you for the additional information. It sounds as if the temperature was OK. It sounds to me as if this is either a case of 'damping off' (fungal attack from soil fungi in the compost) or it's simply a case of the cucumber seedlings getting too leggy too quickly.

If you want to try again I suggest the following:

Invest in some special, new seed sowing compost that is specific for the job exactly the right balance of sand etc.

Only half fill each pot with the compost, sow the seed. Let the seed germinate and grow up to the rim of the pot. Then start to 'earth up' the stem as it grows until you fill the pot, this will provide extra support and stop it from falling over.